The present invention relates to Ni alloy particles for anisotropic conductive film, and a method for producing such Ni alloy particles and an anisotropic conductive film comprising such Ni alloy particles.
Anisotropic conductive films are mostly used for electric connection between electrodes (ITO) of displays such as liquid crystal displays, organic EL displays, etc. for electronics appliances such as personal computers and portable communications equipment and terminals of semiconductors and between electrodes of tape carrier packages (TCP) and electrodes of printed circuit boards (PCB), etc. Anisotropic conductive films are thin films made of resins in which fine conductive particles are dispersed, and inserted between opposing electrodes (electrodes to be connected) and pressed to achieve electric conduction therebetween. Though fine conductive particles are dispersed in the resins with such distance therebetween as to make the films non-conductive, particles trapped between electrodes to be connected by pressing constitute conduction paths. Therefore, only the opposing electrodes are electrically conducted, while keeping insulation between electrodes that should not be connected.
Conventionally used as conductive particles for anisotropic conductive films are resin particles having metal plating. To reduce electric resistance at the time of connection, proposal has recently been made to use as conductive particles powder of Ni, Cu, Au, Ag or alloys thereof, particularly powder of Ni or alloys thereof.
However, not only are metal-plated resin particles expensive, but also the resin particles are insufficient in hardness to destroy or pierce oxide layers on electrodes. Further, the metal-plated resin particles are poor in conductivity only with metal plating, because the resin particles are insulating.
To solve these problems, Japanese Patent Laid-Open No. 8-273440 proposes the use of powder of Ni, Cu, Au, Ag or alloys thereof as conductive particles. Japanese Patent Laid-Open No. 8-273440 describes that among these conductive particles, Au is expensive, Ag causes migration, Cu is so easily oxidized that its conductivity is deteriorated, and Ni is so easily oxidized and hard to deform that it cannot provide stable electric connection when used as conductive particles for anisotropic conductive films, and that Cuxe2x80x94Ag alloy powder produced by a gas atomizing method is most preferable because it is excellent in oxidation resistance and the suppression of migration.
However, alloy powder produced by a gas atomizing method has a relatively large particle size, suffering from the disadvantage that extremely low yield is provided to produce alloy powder having as small a particle size as 10 xcexcm or less, for instance. There is also likelihood of oxidation and migration in the Cuxe2x80x94Ag alloy powder.
To ensure electric conduction in the anisotropic conductive film, there are a method (1) of using low-hardness conductive particles to expand a contact area of the conductive particles with electrodes by the deformation of the conductive particles, and a method (2) of using high-hardness conductive particles to destroy an oxide layer formed on an electrode surface. The proposal of Japanese Patent Laid-Open No. 8-273440 is based on the method (1). It has been found, however, that the method (1) cannot necessarily provide secure electric conduction.
To adopt the method (2), investigation has been conducted with respect to a method for increasing the hardness of conductive particles. It has thus been found that when pure Ni particles less likely to generate migration are used as conductive particles, pure Ni particles are easily oxidized, and neither sufficiently soft to expand the contact area by deformation, nor hard to penetrate an oxide layer on an electrode surface, as described in Japanese Patent Laid-Open No. 8-273440, failing to provide stable electric conduction.
As a result of investigation on the composition of an Ni alloy capable of having high hardness and a method for efficiently producing fine, uniform conductive particles as small as 10 xcexcm or less, which cannot easily be obtained by an atomizing method, it has been found that (1) the addition of a metalloid element is effective for high hardness, (2) Ni alloy particles having a particle size that is as small as 10 xcexcm or less and uniform, which cannot easily be obtained by an atomizing method, can be produced by an electroless reduction method, (3) though Ni alloy particles produced by an electroless reduction method are substantially amorphous and fine with high hardness, they cannot be used for powder for anisotropic conductive films because of high electric resistance, and (4) to have necessary hardness for surely penetrating an oxide layer on an electrode, further increase in hardness should be achieved.
Accordingly, an object of the present invention is to provide Ni alloy particles capable of forming an anisotropic conductive film that provides good electric conduction in contact with electrodes by pressing.
Another object of the present invention is to provide a method for producing such Ni alloy particles.
A further object of the present invention is to provide an anisotropic conductive film comprising such Ni alloy particles.
As a result of intense research in view of the above objects, the inventors have found that high-hardness, low-electric resistance powder for an anisotropic conductive film can be obtained by preparing substantially amorphous Ni alloy particle by an electroless reduction method, and heat-treating the substantially amorphous Ni alloy particle to precipitate a Ni intermetallic compound phase in the alloy structure. The present invention is based on this finding.
Thus, the Ni alloy particle for an anisotropic conductive film according to the present invention is a crystalline Ni alloy particle comprising Ni and a metalloid element and having a structure in which a Ni intermetallic compound phase is precipitated.
In a preferred embodiment of the present invention, the Ni intermetallic compound is Ni3P or Ni3B. The Ni alloy particle is substantially spherical, having a particle size distribution with d90 of 10 xcexcm or less, wherein d90 is defined as a particle size of 90% of powder in an accumulative distribution curve. The Ni alloy particle is coated with Au.
The method for producing a Ni alloy particle for an anisotropic conductive film according to the present invention comprises the steps of preparing substantially amorphous, fine Ni alloy particle by an electroless reduction method, and heat-treating the substantially amorphous, fine Ni alloy particle.
In a preferred embodiment of the present invention, heat treatment is carried out after disintegrating the substantially amorphous Ni alloy particle produced by the electroless reduction method. Further preferably, the heat-treated Ni alloy particle is coated with Au.
The anisotropic conductive film of the present invention comprises crystalline Ni alloy particles uniformly dispersed in a resin, the crystalline Ni alloy particles comprising Ni and a metalloid element and having a structure in which a Ni intermetallic compound phase is precipitated. The content of the Ni alloy particles is preferably 1-20% by weight, more preferably 2-10% by weight, based on the total amount (100% by weight) of the film.